Unconventional resources of oil and gas from a geologic perspective

Overview

For a clear knowing of the unconventional resources, you first have to know the difference between both the Conventional and the Unconventional Reservoirs.

The conventional reservoirs are those which well can be drilled through, so that oil and natural gas can be produced at economic flow rates without large stimulation treatments or any special recovery process.

On the other hand, the unconventional reservoir is one that cannot be produced at economic flow rates or that does not produce economic volumes of oil and gas without assistance from massive stimulation treatments or special recovery processes and technologies.

Think of the difference between a sponge and a piece of clay, it’s easy to squeeze water out of a saturated sponge, that’s a conventional oil and natural gas reservoir; squeezing water out of saturated clay is harder - that's an unconventional one.

Oil and natural gas are the main sources of energy around the world which are being exhausted decade after decade, that have resulted a decline in these conventional resources.

As technologies tackling the petroleum industry are always expanding and developing, introducing a new technology that tackles unconventional reservoirs became a must to increase the world reserve by producing “unconventional” oil and natural gas resources that were previously impossible to do.

And these unconventional reservoirs contain the future of our hydrocarbon supply which is the unconventionalresources.

Unconventional resources Classification and distribution

The classification of unconventional resources

The unconventional resources are classified into: shale gas, shale oil, tight gas, tight oil, coal seam gas/coalbed methane and hydrates, most of them will be tackled through the article from a geologic perspective.

The distribution of unconventional main resources

 Here we have a table showing the distribution of worldwide unconventional-gas resources (after Rogner 1996 , taken from Kawata and Fujita 2001)

 

Coal Bed Methane (CBM)

Formation of coal bed methane

In order to understand the formation of coal bed methane you have firstly to understand the formation of both coal and peat.

Formation of coal

Coal was formed about 400 million years ago from the remains of vegetations that grew at that time, so it’s called a fossil fuel.

Formation of peat

Peat is a soggy, dense material which is formed by accumulation of layers over the remains of dead plants and trees that sank to the bottom of the swampy areas.

Over long periods of time, the changes in the earth's surface caused deposits of sands, clays and other minerals to accumulate, burying the peat underneath

Sandstone and other sedimentary rocks were formed, and the pressure caused by their weight squeezed water out from the peat.

This depth associated with heat, gradually changed the material to coal. Scientists claim that, from 3 to 7 feet of compacted plant matter is required to form 1 foot of bituminous coal.

Formation of coal bed methane

Biogenic methane is produced by anaerobic bacteria in the early stages of coalification.Thermogenic methane is mainly produced during coalification at temperatures ranging from 120 – 150 °C.Contrasting features between CBM and Conventional Gas Reservoirs

1-Gas Composition

Gas produced from coal beds may be initially higher in methane content than the gas produced from conventional reservoirs.

Methane is less adsorbed than ethane and other heavier saturated hydrocarbons; consequently, they may not be as readily desorbed at first.

2-Adsorption

The mechanism by which hydrocarbon gases are stored in the coal reservoir contrasts with the mechanism of the gas storage in conventional reservoirs.

Methane is held to the solid surface of coal by adsorption forces instead of occupying void spaces -as a free gas- between sand grains (only 1-2%).

The adsorption mechanism creates the paradox of high gas storage in a reservoir rock of porosity less than 2.5%.

A clear illustration of the enormous surface area in the micropores of the coal is that 1 lb of coal has a surface area of 55 football fields, or 1 billion sq ft per ton of coal. 

3-Water Production

In the early production life of a well, before methane can be desorbed, the water from natural fractures in the coal must be removed.

The large volumes of water in the first year or two of production, decrease thereafter to relatively small volumes for the remaining life of the well.

4-Rock Physical Properties

Conventional oil and gas formations are inorganic. Organic formations contain CBM; these formations may contain about 10–30% inorganic ash.

5-Gas Flow

For coals, an additional mechanism of gas diffusion through the micropores of the coal matrix is involved, where the mass transport depends upon a methane concentration gradient across the micropores as a driving force.

Distribution of coal bed methane

Distribution of coal bed methane is illustrated by the following table